1. Field of the Invention
The invention relates to high-voltage change-over switches and, more particularly, to those used to provide supplies alternately to at least two X-ray tubes from a single high-voltage generator.
X-ray tubes for medical diagnosis are generally constituted (FIG. 1) like a diode, i.e. with a cathode 11 and an anode 12 or anti-cathode, these two electrodes being enclosed in a vacuum-tight casing 14 that provides for the electrical insulation between these two electrodes. The cathode 11 produces a beam 13 of electrons and the anode receives these electrons on a small area that constitutes a focal spot from which the X-rays are emitted.
When the high supply voltage is applied by a generator 15 to the terminals of the cathode 11 and of the anode 12 so that the cathode is at a negative potential -HT and the anode at a positive potential +HT, a current known as an anode current flows in the circuit through the generator 15 supplying the high voltage. The anode current goes through the space between the cathode and the anode in the form of the beam 13 of electrons which impinge on the focal spot.
A small proportion of the energy used to produce the electron beam 13 is converted into X-rays, the rest of this energy being converted into heat. Hence, in view also of the high instantaneous power (in the range of 100 KW) brought into play and of the dimensions of the focal spot (in the range of one millimetre), manufacturers have long been making X-ray tubes with rotating anodes where the anode is made to rotate in order to distribute the heat flux on a ring called a focal ring, with an area far greater than that of the focal spot, the value of this approach increasing concomitantly with the rise in rotational speed (generally between 3,000 and 12,000 rpm).
The standard type of rotating anode has the general shape of a disk with an axis of symmetry 16 about which it is made to rotate by means of an electrical motor 17; the electrical motor has a stator 18 located outside the casing 14 and a rotor 19 mounted in the casing 14 of the X-ray tube and positioned along the axis of symmetry 16, the rotor being mechanically fixed to the anode by means of a supporting shaft 20.
2. Description of the Prior Art
The high-voltage generator 15, which gives a voltage ranging from 50 to 160 kilovolts between the -HT and +HT terminals, is a major, bulky and costly element of a radiological apparatus. Thus, in radiology installations comprising several X-ray tubes, there is provision for using only one high-voltage connector which is connected to the different X-ray tubes by means of a high-voltage change-over switch, the schematic diagram of which is given in FIG. 2 which depicts the case of a change-over switch 21 for the supply of two tubes A and B. This change-over switch 21 has two input terminals 22 and 23, respectively connected to the +HT and -HT terminals of the high-voltage generators, and two pairs of output terminals 24, 25 and 26, 27 respectively connected to the tubes A and B. The switching over is done by means of two rotary arms 28 and 29 connected on one side (contact elements 22' and 23') respectively to the input terminals 22 and 23 and, on the other side, either to the output terminals 24 and 25 (contact elements 24' and 25') for a first position of the arms (when supplying the tube A) or to the output terminals 26 and 27 (contact elements 26' and 27') for a second position of the arms (when supplying the tube B). With a mechanism such as this, it is necessary for the distances between the different contact elements to be great enough to prevent conduction by electrical arcing. Thus, in dry air, the distances should be of the order of several centimetres, for example 15 centimetres for 150 kilovolts, which results in change-over switches that are large-sized and hence very bulky. Thus, to reduce this bulk, it is usual to place the change-over switch or switches in a chamber filled with insulating oil, the disruptive voltage of which is equal to or greater than 10 kilovolts per millimetre instead of one kilovolt per millimetre in dry air. This leads, naturally, to greater compactness but entails the necessity of using an oil-filled chamber.